The EGVs provide power for the lighting, aircon, PA systems etc.
There were EGVs for the Mk2, Mk3s. The Mk3 pushpull DVTs had a small generator built into them that could power them selves and up to a max of 5 other standard Mk3.
The 201s on the DD Enterprise sets provide HEP, head end power. They run at 900rpm when idle when in this mode and that's why they are louder when on the DD stock. Around 900hp is taken from the 201 to provide power to the train leaving it with about the same power as an 071 to haul the train. HEP puts alot to strain on the 201s which is why they fail alot on the Enterprise and the locos are swapped out regularly to give them a break. With the Mk3s being withdrawn, 4 Mk3 EGVs have been modified to work with the DD stock to cut down on 201 failures. They have been in testing for 2 years and the first one entered service last September. In mid December the 3 sets had an EGV each but as of last week at least one has been removed from service. They seem to be having teething problems with them.
The Mk4 DVTs are also EGVs or DVGTs.